Fix travis config

Current environment no longer includes xmllint by default

Author: noniq

.travis.yml

@@ -1,3 +1,8 @@
 script:
   - xmllint --valid *.plist */*.tm* >/dev/null
   - ruby -c */*/*.rb
+addons:
+  apt:
+    packages:
+      # Needed for `xmllint`.
+      - libxml2-utils